In systems providing such transmission, it is desirable to reduce the spectrum width of the signal, i.e. the spectrum bandwidth occupied by the signal to be transmitted, to as narrow a width as possible without increasing the transmission error rate to a problematic extent. In particular, reduced spectrum width can make it possible to increase the number of transmission spectrum channels carried by the same wave.
A known "duobinary" encoding method can be used to achieve the above-mentioned reduction. It is particularly applicable to transmission systems in which the carrier wave is a light wave guided by an optical fiber.
In that case, reducing the spectrum bandwidth of the signal is especially desirable because optical fibers often exhibit chromatic dispersion and because such dispersion deforms the transmitted signal, which deformation increases with increasing signal spectrum width, and thus increases the error rate.
An improved example of that encoding method has been proposed, and it is particularly advantageous when the carrier wave is a light wave guided by an optical fiber because it enables the intensity of the carrier wave to be given two values only. That improved example may be referred to as "phase inversion duobinary" encoding. It causes the carrier wave to have an intensity and a phase that are substantially constant during each of the groups of zero bits or of one bits making up the input signal, each group of zero bits extending between two one bits and being constituted by at least one zero bit and no one bits, and each group of one bits extending between two zero bits and being constituted by at least one one bit and no zero bits. Said intensity is equal to a nominal intensity during the groups of one bits. During the groups of zero bits, it is equal to the quotient of the nominal intensity divided by an extinction ratio TX which it is desirable to make as large as possible. Said phase constitutes a reference phase during the groups of zero bits, and it has a phase shift relative to the reference phase during the groups of one bits. This shift is associated with the group. The sign of shift is reversed between two consecutive groups of one bits when and only when the group of zero bits separating the two groups of one bits comprises an odd number of bits. And its amplitude is set so as to be equal to about 90 degrees.
This set value is inherent to the code. According to the known theory that was used to develop the code, when two consecutive groups of one bits are separated from each other by an odd number of bit periods, the looked-for reduction in the spectrum width of the signal is obtained and results from the combination of two facts. The first fact is that, during the two groups of one bits, a modulation factor affecting the field of the carrier wave has two mutually symmetrical values such as 1 and -1. The second fact is that a cumulative phase shift of 2.times.90=180 degrees of the carrier wave between the two groups makes it possible to give two such symmetrical values to the modulation factor while giving the same value to the intensity of the carrier wave throughout all of the groups of one bits.
The phase inversion duobinary code is described in an article entitled "Reduced bandwidth optical digital intensity modulation with improved chromatic dispersion tolerance", A. J. Price et al., IEEE Electronics Letters, Vol. 31, No. 1, pages 58-59, January 1995).
In known manner, it requires a modulator to be used that satisfies two conditions. The first condition is that the modulator makes it possible to achieve an extinction ratio that is high enough for the intensity of the carrier wave to be considered as being zero during the groups of zero bits.
The second condition is that the modulator is suitable for performing the cumulative phase shift of 180.degree. that the code requires in certain cases between two consecutive groups of one bits.
In the optical field, a type of modulator is known that satisfies both conditions: it is the Mach-Zehnder type of interferometric modulator. That type of known modulator suffers from the drawbacks of ageing quickly, of being costly, bulky, and/or difficult to integrate with other optical components required in a transmission system.